Modern birds are characterised
by feathers, a beak with no teeth, the laying of hard-shelled eggs,
a high metabolic
rate, a four-chambered heart, and a lightweight but strong skeleton.
All birds have forelimbs modified as wings and most can fly, with
some exceptions including ratites, penguins, and a number of
diverse endemic
island species. Birds also have unique digestive
and respiratory
systems that are highly adapted for flight. Some birds,
especially corvids and
parrots, are among the
most intelligent animal species; a number of bird species have been
observed manufacturing and using tools, and many social species
exhibit cultural
transmission of knowledge across generations.

Many species undertake long distance annual
migrations,
and many more perform shorter irregular movements. Birds are
social; they communicate using visual signals and through calls and
songs,
and participate in social behaviours including cooperative
breeding and hunting, flocking,
and mobbing
of predators. The vast majority of bird species are socially
monogamous, usually for one breeding season at a time,
sometimes for years, but rarely for life. Other species have
breeding systems that are polygynous ("many females") or,
rarely, polyandrous
("many males"). Eggs are usually laid in a nest and incubated
by the parents. Most birds have an extended period of parental care
after hatching.

Many species are of economic importance, mostly
as sources of food acquired through hunting or farming. Some
species, particularly songbirds and parrots, are popular as pets.
Other uses include the harvesting of guano (droppings) for use as a
fertiliser. Birds
figure
prominently in all aspects of human culture from religion to
poetry to popular music. About 120–130 species have become extinct as a result of human
activity since the 17th century, and hundreds more before then.
Currently about 1,200 species of birds are threatened with
extinction by human activities, though efforts are underway to
protect
them.

All modern birds lie within the subclassNeornithes,
which has two subdivisions: the Paleognathae,
containing mostly flightless birds like ostriches, and the wildly
diverse Neognathae,
containing all other birds. although Livezey & Zusi assigned
them "cohort" rank. to 10,050.

Dinosaurs and the origin of birds

Fossil evidence and
intensive biological analyses have demonstrated beyond any
reasonable doubt that birds are theropoddinosaurs. More
specifically, they are members of Maniraptora, a
group of theropods which includes dromaeosaurs and oviraptorids,
among others. As scientists discover more non-avian theropods that
are closely related to birds, the previously clear distinction
between non-birds and birds has become blurred. Recent discoveries
in the Liaoning Province
of northeast
China, which demonstrate that many small theropod
dinosaurs had feathers, contribute to this ambiguity.

The consensus view in contemporary paleontology is that the
birds, Aves,
are the closest relatives of the deinonychosaurs, which
include dromaeosaurids and
troodontids.
Together, these three form a group called Paraves. The
basaldromaeosaurMicroraptor has
features which may have enabled it to glide or fly. The most basal
deinonychosaurs are very small. This evidence raises the
possibility that the ancestor of all paravians may have been
arboreal, and/or may
have been able to glide.

The Late
JurassicArchaeopteryx
is well-known as one of the first transitional
fossils to be found and it provided support for the theory of
evolution in the late
19th century. Archaeopteryx
has clearly reptilian characters: teeth, clawed fingers, and a
long, lizard-like tail, but it has finely preserved wings with
flight feathers identical to those of modern birds. It is not
considered a direct ancestor of modern birds, but is the oldest and
most primitive member of Aves or Avialae, and it is
probably closely related to the real ancestor. It has even been
suggested that Archaeopteryx was a dinosaur that was no more
closely related to birds than were other dinosaur groups, and that
Avimimus
was more likely to be the ancestor of all birds than
Archaeopteryx.

Alternative theories and controversies

There have been many
controversies in the study of the origin of birds. Early
disagreements included whether birds evolved from dinosaurs or more primitive
archosaurs. Within the
dinosaur camp there were disagreements as to whether ornithischian or theropod dinosaurs were the
more likely ancestors. Although ornithischian (bird-hipped)
dinosaurs share the hip structure of modern birds, birds are
thought to have originated from the saurischian (lizard-hipped)
dinosaurs, and therefore evolved their hip structure independently.
In fact, a bird-like hip structure evolved a third time among a
peculiar group of theropods known as the Therizinosauridae.

Scientists Larry Martin
and Alan
Feduccia believe that birds are not dinosaurs, but that birds
evolved from early archosaurs like Longisquama.
The majority of their publications argued that the similarities
between birds and maniraptoran dinosaurs were
convergent, and that the two were unrelated. In the late 1990s the
evidence that birds were maniraptorans became almost
indisputable, so Martin and Feduccia adopted a modified version of
a hypothesis by dinosaur artist Gregory S.
Paul; where maniraptorans are secondarily flightless birds but,
in their version, birds evolved directly from Longisquama.
Thus birds are still not dinosaurs, but neither are most of the
known species that are currently classified as theropod dinosaurs. Maniraptorans
are, instead, flightless, archosaurian, birds. This
theory is contested by most paleontologists. The
features cited as evidence of flightlessness are interpreted by
mainstream paleontologists as exaptations, or
"pre-adaptations", that maniraptorans inherited from
their common ancestor with birds.

Protoavis
texensis, was described in 1991 as a bird older than Archaeopteryx.
Critics have indicated that the fossil is poorly preserved,
extensively reconstructed, and may be a chimera (made up of fossilized
bones from several different kinds of animals). The braincase is
most likely that of a very early coelurosaur

Early evolution of birds

Basal bird phylogeny simplified
after Chiappe, 2007 Birds diversified into a wide variety of forms
during the Cretaceous
Period. One order of Mesozoic seabirds, the Hesperornithiformes,
became so well adapted to hunting fish in marine environments that
they lost the ability to fly and became primarily aquatic. Despite
their extreme specialisations, the Hesperornithiformes represent
some of the closest relatives of modern birds. and is split into
two superorders, the Paleognathae
and Neognathae. The
paleognaths include the tinamous of Central
and South
America and the ratites. The basal divergence
from the remaining Neognathes was that of the Galloanserae,
the superorder containing the Anseriformes
(ducks, geese, swans and screamers) and the Galliformes
(the pheasants,
grouse, and their allies,
together with the mound
builders and the guans and
their allies). The dates for the splits are much debated by
scientists. It is agreed that the Neornithes evolved in the
Cretaceous, and that the split between the Galloanseri from other
Neognathes occurred before the
K–T extinction event, but there are different opinions about
whether the radiation
of the remaining Neognathes occurred before or after the extinction
of the other dinosaurs. This disagreement is in part caused by a
divergence in the evidence; molecular dating suggests a Cretaceous
radiation, while fossil
evidence supports a Tertiary
radiation. Attempts to reconcile the molecular and fossil evidence
have proved controversial.

The classification of birds is a contentious
issue. Sibley and
Ahlquist's
Phylogeny and Classification of Birds (1990) is a landmark work on
the classification of birds, although it is frequently debated and
constantly revised. Most evidence seems to suggest that the
assignment of orders is accurate, but scientists disagree about the
relationships between the orders themselves; evidence from modern
bird anatomy, fossils and DNA have all been brought to bear on the
problem, but no strong consensus has emerged. More recently, new
fossil and molecular evidence is providing an increasingly clear
picture of the evolution of modern bird orders.

Modern bird orders

This is a list of the taxonomic orders in the
subclass Neornithes, or modern birds. This list uses the
traditional classification (the so-called Clements
order), revised by the Sibley-Monroe classification. The list of
birds gives a more detailed summary of the orders, including
families.

The radically different Sibley-Monroe
classification (Sibley-Ahlquist
taxonomy), based on molecular data, found widespread adoption
in a few aspects, as recent molecular, fossil, and anatomical
evidence supported the Galloanserae
for example. ]]

Birds live and breed in most terrestrial habitats
and on all seven continents, reaching their southern extreme in the
Snow
Petrel's breeding colonies up to inland in Antarctica. The
highest bird diversity occurs in
tropical regions. It was earlier thought that this high diversity
was the result of higher speciation rates in the
tropics, however recent studies found higher speciation rates in
the high latitudes that were offset by greater extinction rates than in the
tropics. Several families of birds have adapted to life both on the
world's oceans and in them, with some seabird species coming ashore
only to breed and some penguins have been recorded
diving up to .

Many bird species have established breeding
populations in areas to which they have been introduced
by humans. Some of these introductions have been deliberate; the
Ring-necked
Pheasant, for example, has been introduced around the world as
a game
bird. Others have been accidental, such as the establishment of
wild Monk
Parakeets in several North American cities after their escape
from captivity. Some species, including Cattle
Egret, Yellow-headed
Caracara and Galah, have spread
naturally far beyond their original ranges as agricultural
practices created suitable new habitat.

Anatomy

Compared with other vertebrates, birds have a
body
plan that shows many unusual adaptations, mostly to facilitate
flight.

The skeleton consists of very lightweight bones.
They have large air-filled cavities (called pneumatic cavities)
which connect with the respiratory
system. The skull bones are fused and do not show cranial
sutures. The orbits
are large and separated by a bony septum. The spine
has cervical, thoracic, lumbar and caudal regions with the number
of cervical (neck) vertebrae highly variable and especially
flexible, but movement is reduced in the anterior thoracic
vertebrae and absent in the later vertebrae. The last few are fused
with the pelvis to form
the synsacrum.

Like the reptiles, birds are primarily
uricotelic, that is, their kidneys extract nitrogenous
wastes from their bloodstream and excrete it as uric acid
instead of urea or ammonia. Uric acid is excreted
along with feces as a semisolid waste since birds do not have a
separate bladder or uretral opening. However, birds such as
hummingbirds can be facultatively ammonotelic, excreting most of
the nitrogenous wastes as ammonia. They also excrete creatine, rather than creatinine like mammals. The
cloaca is a multi-purpose opening: waste is expelled through it,
birds mate by joining
cloaca, and females lay eggs from it. In addition, many species
of birds regurgitate pellets.
The digestive
system of birds is unique, with a crop for
storage and a gizzard
that contains swallowed stones for grinding food to compensate for
the lack of teeth. Most birds are highly adapted for rapid
digestion to aid with flight. Some migratory birds have the
additional ability to reduce parts of the intestines prior to
migration.

Birds have one of the most complex respiratory
systems of all animal groups. Sound production is achieved
using the syrinx,
a muscular chamber with several tympanic membranes which is
situated at the lower end of the trachea, from where it separates.
The bird's heart has four chambers and the right aortic arch gives
rise to systemic
circulation (unlike in the mammals where the left arch is
involved).

The nervous
system is large relative to the bird's size. New World
vultures and tubenoses. The avian visual
system is usually highly developed. Water birds have special
flexible lenses, allowing accommodation for vision in air and
water. This allows them to perceive ultraviolet light, which is
involved in courtship. Many birds show plumage patterns in
ultraviolet that are invisible to the human eye; some birds whose
sexes appear similar to the naked eye are distinguished by the
presence of ultraviolet reflective
patches on their feathers. Male Blue Tits have
an ultraviolet reflective crown patch which is displayed in
courtship by posturing and raising of their nape feathers.
Ultraviolet light is also used in foraging—kestrels have been shown to
search for prey by detecting the UV reflective urine trail marks
left on the ground by rodents. The eyelids of a bird are not used
in blinking. Instead the eye is lubricated by the nictitating
membrane, a third eyelid that moves horizontally. The
nictitating membrane also covers the eye and acts as a contact lens
in many aquatic birds. Birds with eyes on the sides of their heads
have a wide visual
field, while birds with eyes on the front of their heads, such
as owls, have binocular
vision and can estimate the depth of field. The avian ear lacks external pinnae
but is covered by feathers, although in some birds, such as the
Asio, Bubo and
Otusowls, these feathers form
tufts which resemble ears. The inner ear has a cochlea, but it is not spiral as
in mammals.

A few species are able to use chemical defenses
against predators; some Procellariiformes
can eject an unpleasant oil against
an aggressor, and some species of pitohuis from New Guinea
secrete a powerful neurotoxin in their skin and
feathers.

Birds have two sexes: male and female. Birds' sex
is determined by
Z and W sex chromosomes, rather than the X and Y chromosomes
seen in mammals. Males carry two Z chromosomes (ZZ), and females
carry a W chromosome and a Z chromosome (WZ).

Feathers and plumage

Feathers are a
feature unique to birds. They facilitate flight,
provide insulation that aids in thermoregulation, and
are used in display, camouflage, and signaling. and sex.

Plumage is regularly moulted; the standard plumage of a
bird that has moulted after breeding is known as the "non-breeding"
plumage, or – in the Humphrey-Parkes
terminology – "basic" plumage; breeding plumages or variations
of the basic plumage are known under the Humphrey-Parkes system as
"alternate" plumages. Moulting is annual in most species, although
some may have two moults a year, and large birds of prey may moult
only once every few years. Moulting patterns vary across species.
Some drop and regrow wing flight
feathers, starting sequentially from the outermost feathers and
progressing inwards (centripetal), while others replace feathers
starting from the innermost ones (centrifugal). A small number of
species, such as ducks and geese, lose all of their flight feathers
at once, temporarily becoming flightless. Centripetal moults of
tail feathers are seen for example in the Phasianidae.
Centrifugal moult is seen, for instance, in the tail feathers of
woodpeckers and
treecreepers,
although it begins with the second innermost pair of tail-feathers
and finishes with the central pair of feathers so that the bird
maintains a functional climbing tail. The general pattern seen in
passerines is that the
primaries are replaced outward, secondaries inward, and the tail
from center outward. Before nesting, the females of most bird
species gain a bare brood patch
by losing feathers close to the belly. The skin there is well
supplied with blood vessels and helps the bird in incubation.

Feathers require maintenance and birds preen or
groom them daily, spending an average of around 9% of their daily
time on this. The bill is used to brush away foreign particles and
to apply waxy secretions
from the uropygial
gland; these secretions protect the feathers' flexibility and
act as an antimicrobial agent, inhibiting the growth of
feather-degrading bacteria. This may be
supplemented with the secretions of formic acid
from ants, which birds receive through a behaviour known as
anting, to remove feather parasites.

Scales

The scales of birds are composed of the same keratin
as beaks, claws, and spurs. They are found mainly on the toes and
metatarsus, but may
be found further up on the ankle in some birds. Most bird scales do
not overlap significantly, except in the cases of kingfishers and woodpeckers.

Bird embryos begin development with smooth skin.
On the feet, the corneum,
or outermost layer, of this skin may keratinize, thicken and form
scales. These scales can be organized into;

Cancella - minute scales which are really just a thickening and
hardening of the skin, crisscrossed with shallow grooves.

Reticula - small but distinct, separate, scales. Found on the
lateral and medial surfaces (sides) of the
chicken metatarsus.

Scutella - scales that are not quite as large as scutes, such
as those found on the caudal, or hind part, of the
chicken metatarsus.

The rows of scutes on the anterior of the metatarsus can be called an
acrometatarsium or acrotarsium.

Feathers can be intermixed with scales on some
birds' feet. Feather follicles can lie between scales or even
directly beneath them, in the deeper dermis layer of the skin. In this
last case, feathers may emerge directly through scales, and be
encircled at the plane of emergence entirely by the keratin of the
scale.

The scales of birds are thought to be homologous
to those of reptiles and mammals.

Flight

Most birds can
fly, which distinguishes them from almost all other
vertebrates. Flight is the primary means of locomotion for most
bird species and is used for breeding, feeding, and predator
avoidance and escape. Birds have various adaptations for flight,
including a lightweight skeleton, two large flight muscles (the
pectoralis—accounting for 15% of the total mass of the bird—and the
supracoracoideus), and a modified forelimb (wing) that serves as an aerofoil. Flightlessness often
arises in birds on isolated islands, probably due to limited
resources and the absence of land predators. Though flightless,
penguins use similar musculature and movements to "fly" through the
water, as do auks, shearwaters and dippers.

Behaviour

Most birds are diurnal,
but some birds, such as many species of owls and nightjars, are nocturnal or crepuscular (active during
twilight hours), and many coastal waders feed when the tides are
appropriate, by day or night.

Diet and feeding

Birds' diets are varied and often include
nectar,
fruit, plants, seeds, carrion, and various small
animals, including other birds. Kiwis and shorebirds with long bills
probe for invertebrates; shorebirds' varied bill lengths and
feeding methods result in the separation of ecological
niches. Loons, diving ducks,
penguins and auks pursue their prey underwater,
using their wings or feet for propulsion, Geese and dabbling
ducks are primarily grazers. Some species, including frigatebirds, gulls, and skuas, engage in kleptoparasitism,
stealing food items from other birds. Kleptoparasitism is thought
to be a supplement to food obtained by hunting, rather than a
significant part of any species' diet; a study of Great
Frigatebirds stealing from Masked
Boobies estimated that the frigatebirds stole at most 40% of
their food and on average stole only 5%. Other birds are scavengers; some of these,
like vultures, are
specialised carrion eaters, while others, like gulls, corvids, or other birds of prey,
are opportunists.

Migration

Many bird species migrate to take advantage of
global differences of seasonal temperatures, therefore
optimising availability of food sources and breeding habitat. These
migrations vary among the different groups. Many landbirds,
shorebirds, and
waterbirds undertake
annual long distance migrations, usually triggered by the length of
daylight as well as weather conditions. These birds are
characterised by a breeding season spent in the temperate or arctic/antarctic regions and a
non-breeding season in the tropical regions or opposite
hemisphere. Before migration, birds substantially increase body
fats and reserves and reduce the size of some of their organs.
although the Bar-tailed
Godwit is capable of non-stop flights of up to . Seabirds also
undertake long migrations, the longest annual migration being those
of Sooty
Shearwaters, which nest in New Zealand
and Chile and
spend the northern summer feeding in the North Pacific off Japan, Alaska and California, an
annual round trip of . Other seabirds disperse after breeding,
travelling widely but having no set migration route. Albatrosses
nesting in the Southern Ocean often undertake circumpolar trips
between breeding seasons. Some bird species undertake shorter
migrations, travelling only as far as is required to avoid bad
weather or obtain food. Irruptive
species such as the boreal finches are one such group and can
commonly be found at a location in one year and absent the next.
This type of migration is normally associated with food
availability. Species may also travel shorter distances over part
of their range, with individuals from higher latitudes travelling
into the existing range of conspecifics; others undertake partial
migrations, where only a fraction of the population, usually
females and subdominant males, migrates. Partial migration can form
a large percentage of the migration behaviour of birds in some
regions; in Australia, surveys found that 44% of non-passerine
birds and 32% of passerines were partially migratory. Altitudinal
migration is a form of short distance migration in which birds
spend the breeding season at higher altitudes elevations and move
to lower ones during suboptimal conditions. It is most often
triggered by temperature changes and usually occurs when the normal
territories also become inhospitable due to lack of food. Some
species may also be nomadic, holding no fixed territory and moving
according to weather and food availability. Parrots as a
family
are overwhelmingly neither migratory nor sedentary but considered
to either be dispersive, irruptive, nomadic or undertake small and
irregular migrations.

The ability of birds to return to precise
locations across vast distances has been known for some time; in an
experiment conducted in the 1950s a Manx
Shearwater released in Boston
returned to its colony in Skomer, Wales within 13 days,
a distance of . Birds navigate during migration using a variety of
methods. For diurnal
migrants, the sun is used to
navigate by day, and a stellar compass is used at night. Birds that
use the sun compensate for the changing position of the sun during
the day by the use of an internal
clock. These are backed up in some species by their ability to
sense the Earth's geomagnetism through
specialised photoreceptors.

Communication

Birds sometimes use plumage to assess and assert
social dominance, to display breeding condition in sexually
selected species, or to make threatening displays, as in the
Sunbittern's
mimicry of a large predator to ward off hawks and protect young chicks.
Variation in plumage also allows for the identification of birds,
particularly between species. Visual communication among birds may
also involve ritualised displays, which have developed from
non-signalling actions such as preening, the adjustments of feather
position, pecking, or other behaviour. These displays may signal
aggression or submission or may contribute to the formation of
pair-bonds. males' breeding success may depend on the quality of
such displays.

Bird calls
and songs, which are produced in the syrinx,
are the major means by which birds communicate with sound. This communication can be
very complex; some species can operate the two sides of the syrinx
independently, allowing the simultaneous production of two
different songs. bond formation, the claiming and maintenance of
territories,), and the warning of other birds of potential
predators, sometimes with specific information about the nature of
the threat. Some birds also use mechanical sounds for auditory
communication. The Coenocoryphasnipes of New Zealand
drive air through their feathers, woodpeckers drum
territorially,

Flocking and other associations

While some birds are
essentially territorial or live in small family groups, other birds
may form large flocks. The
principal benefits of flocking are safety in
numbers and increased foraging efficiency. Costs of flocking
include bullying of socially subordinate birds by more dominant
birds and the reduction of feeding efficiency in certain
cases.

Resting and roosting

The high metabolic rates of birds
during the active part of the day is supplemented by rest at other
times. Sleeping birds often use a type of sleep known as vigilant
sleep, where periods of rest are interspersed with quick
eye-opening 'peeks', allowing them to be sensitive to disturbances
and enable rapid escape from threats. Swifts have been
widely believed to be able to sleep while flying; however, this has
not been confirmed by experimental evidence. However, there may be
certain kinds of sleep which are possible even when in flight. Some
birds have also demonstrated the capacity to fall into slow-wave
sleep one hemisphere
of the brain at a time. The birds tend to exercise this ability
depending upon its position relative to the outside of the flock.
This may allow the eye opposite the sleeping hemisphere to remain
vigilant for predators
by viewing the outer margins of the flock. This adaptation is also
known from marine
mammals. Communal roosting is common because it lowers the
loss of body
heat and decreases the risks associated with predators.
Roosting sites are often chosen with regard to thermoregulation and
safety.

Many sleeping birds bend their heads over their
backs and tuck their bills
in their back feathers, although others place their beaks among
their breast feathers. Many birds rest on one leg, while some may
pull up their legs into their feathers, especially in cold weather.
Perching birds have a tendon locking mechanism that helps them hold
on to the perch when they are asleep. Many ground birds, such as
quails and pheasants, roost in trees. A few parrots of the genus
Loriculus
roost hanging upside down. Some hummingbirds go into a
nightly state of torpor
accompanied with a reduction of their metabolic rates. This
physiological
adaptation shows nearly a hundred other species, including
owlet-nightjars,
nightjars, and woodswallows. One species,
the Common
Poorwill, even enters a state of hibernation. Birds do not
have sweat glands, but they may cool themselves by moving to shade,
standing in water, panting, increasing their surface area,
fluttering their throat or by using special behaviours like
urohydrosis to cool
themselves.

Breeding

Social systems

Ninety-five percent of bird species are
socially
monogamous. These species pair for at least the length of the
breeding season or—in some cases—for several years or until the
death of one mate. Monogamy allows for biparental
care, which is especially important for species in which
females require males' assistance for successful brood-rearing.
Among many socially monogamous species, extra-pair copulation
(infidelity) is common. Such behaviour typically occurs between
dominant males and females paired with subordinate males, but may
also be the result of forced copulation in ducks and other anatids. For females, possible
benefits of extra-pair copulation include getting better genes for
her offspring and insuring against the possibility of infertility
in her mate. Males of species that engage in extra-pair copulations
will closely guard their mates to ensure the parentage of the
offspring that they raise.

Other mating systems, including polygyny, polyandry, polygamy, polygynandry, and promiscuity, also occur.
Most displays are rather simple and involve some type of song. Some
displays, however, are quite elaborate. Depending on the species,
these may include wing or tail drumming, dancing, aerial flights,
or communal lekking.
Females are generally the ones that drive partner selection,
although in the polyandrous phalaropes, this is reversed:
plainer males choose brightly coloured females. Courtship feeding,
billing and
allopreening are commonly performed between partners, generally
after the birds have paired and mated.

Bird eggs are usually laid in a nest. Most
species create somewhat elaborate nests, which can be cups, domes,
plates, beds scrapes, mounds, or burrows. Some bird nests, however,
are extremely primitive; albatross nests are no more
than a scrape on the ground. Most birds build nests in sheltered,
hidden areas to avoid predation, but large or colonial birds—which
are more capable of defence—may build more open nests. During nest
construction, some species seek out plant matter from plants with
parasite-reducing toxins to improve chick survival, and feathers
are often used for nest insulation. The warmth for the incubation
of the eggs of megapodes comes from the sun,
decaying vegetation or volcanic sources. Incubation periods range
from 10 days (in woodpeckers, cuckoos and passerine birds) to over 80
days (in albatrosses and kiwis). At the other extreme, many
seabirds have extended periods of parental care, the longest being
that of the Great
Frigatebird, whose chicks take up to six months to fledge and are fed by the parents
for up to an additional 14 months. In some species, both parents
care for nestlings and fledglings; in others, such care is the
responsibility of only one sex. In some species, other
members of the same species—usually close relatives
of the breeding pair, such as offspring from previous
broods—will help with the raising of the young. Such
alloparenting is particularly common among the Corvida, which
includes such birds as the true crows, Australian
Magpie and Fairy-wrens, but
has been observed in species as different as the Rifleman
and Red
Kite. Among most groups of animals, male parental care is rare.
In birds, however, it is quite common—more so than in any
other vertebrate class.

The point at which chicks fledge varies dramatically. The
chicks of the Synthliboramphus
murrelets, like the Ancient
Murrelet, leave the nest the night after they hatch, following
their parents out to sea, where they are raised away from
terrestrial predators. Some other species, such as ducks, move
their chicks away from the nest at an early age. In most species,
chicks leave the nest just before, or soon after, they are able to
fly. The amount of parental care after fledging varies; albatross
chicks leave the nest on their own and receive no further help,
while other species continue some supplementary feeding after
fledging. Chicks may also follow their parents during their first
migration.

Brood parasites

Brood
parasitism, in which an egg-layer leaves her eggs with another
individual's brood, is more common among birds than any other type
of organism. After a parasitic bird lays her eggs in another bird's
nest, they are often accepted and raised by the host at the expense
of the host's own brood. Brood parasites may be either obligate
brood parasites, which must lay their eggs in the nests of other
species because they are incapable of raising their own young, or
non-obligate brood parasites, which sometimes lay eggs in the nests
of conspecifics to
increase their reproductive output even though they could have
raised their own young. One hundred bird species, including
honeyguides, icterids, estrildid
finches and ducks,
are obligate parasites, though the most famous are the cuckoos.

Ecology

Birds occupy a wide range of ecological positions.
Plants and pollinating birds often coevolve, and in some cases
a flower's primary pollinator is the only species capable of
reaching its nectar.

Birds are often important to island ecology.
Birds have frequently reached islands that mammals have not; on
those islands, birds may fulfill ecological roles typically played
by larger animals. For example, in New Zealand the moas were important browsers, as are
the Kereru
and Kokako
today. Nesting seabirds
may also affect the ecology of islands and surrounding seas,
principally through the concentration of large quantities of
guano, which may enrich
the local soil and the surrounding seas.

Relationship with humans

Since birds are highly visible and common
animals, humans have had a relationship with them since the dawn of
man. Sometimes, these relationships are mutualistic, like
the cooperative honey-gathering among honeyguides and African
peoples such as the Borana.
Other times, they may be commensal, as
when species such as the House
Sparrow have benefited from human activities. Several bird
species have become commercially significant agricultural pests,
and some pose an aviation
hazard. Human activities can also be detrimental, and have
threatened numerous bird species with extinction.

Economic importance

Domesticated birds raised for meat and
eggs, called poultry,
are the largest source of animal protein eaten by humans; in 2003,
76 million tons of poultry and 61 million tons of eggs were
produced worldwide. Chickens account for much of human poultry
consumption, though turkeys, ducks, and geese are also relatively
common. Many species of birds are also hunted for meat. Bird
hunting is primarily a recreational activity except in extremely
undeveloped areas. The most important birds hunted in North and
South America are waterfowl; other widely hunted birds include
pheasants, wild
turkeys, quail,
doves, partridge, grouse, snipe, and woodcock. Muttonbirding
is also popular in Australia and New Zealand. Though some hunting,
such as that of muttonbirds, may be sustainable, hunting has led to
the extinction or endangerment of dozens of species.

Other commercially valuable products from birds
include feathers (especially the down of geese and ducks), which are
used as insulation in clothing and bedding, and seabird feces
(guano), which is a
valuable source of phosphorus and nitrogen. The War of
the Pacific, sometimes called the Guano War, was fought in part
over the control of guano deposits.

Religion, folklore and culture

Birds play prominent and
diverse roles in folklore, religion, and popular culture. In
religion, birds may serve as either messengers or priests and
leaders for a deity, such
as in the Make-make
religion in which the Tangata manu
of Easter
Island served as chiefs, or as attendants, as in the case of
Hugin and
Munin, two Common
Ravens who whispered news into the ears of the Norse godOdin. They may
also serve as religious symbols, as when Jonah (Hebrew:
יוֹנָה, dove) embodied the
fright, passivity, mourning, and beauty traditionally associated
with doves. Birds have themselves been deified, as in the case of
the Common
Peacock, which is perceived as Mother Earth by the Dravidians
of India.
Some birds have also been perceived as monsters, including the
mythological Roc and
the Māori's
legendary Pouākai, a giant bird capable of snatching humans.

Perceptions of various bird species often vary
across cultures. Owls are associated
with bad luck, witchcraft, and death in
parts of Africa, but are
regarded as wise across much of Europe. Hoopoes were
considered sacred in Ancient
Egypt and symbols of virtue in Persia, but were
thought of as thieves across much of Europe and harbingers of war
in Scandinavia.

Conservation

Though human activities have allowed the
expansion of a few species, such as the Barn Swallow
and European
Starling, they have caused population decreases or extinction in many other
species. Over a hundred bird species have gone extinct in
historical times, although the most dramatic human-caused avian
extinctions, eradicating an estimated 750–1800 species, occurred
during the human colonisation of Melanesian,
Polynesian, and
Micronesian
islands. Many bird populations are declining worldwide, with 1,221
species listed as threatened
by Birdlife
International and the IUCN in 2007. The most
commonly cited human threat to birds is habitat loss. Other threats
include overhunting, accidental mortality due to structural
collisions or long-line
fishingbycatch,
pollution (including oil spills and pesticide use), competition and
predation from nonnative invasive
species, and climate change. Governments and conservation
groups work to protect birds, either by passing laws that preserve
and restore
bird habitat or by establishing captive
populations for reintroductions. Such projects have produced
some successes; one study estimated that conservation efforts saved
16 species of bird that would otherwise have gone extinct between
1994 and 2004, including the California
Condor and
Norfolk Island Green Parrot.

Sora
Searchable online research archive; Archives of the following
ornithological journals The Auk, Condor,
Journal of Field Ornithology, North American Bird Bander, Studies
in Avian Biology, Pacific Coast Avifauna, and the
Wilson Bulletin.